In camera tracking how do i figure out the settings for the cannon 6d
Optical center should be the center of your image height/2 , width/2 ,  unless you have cropped the image. Then it should be the original center of your image. There is button for this labeled center. This is relevant to accounting for lens distortion.  The 

Pixel aspect ratio should be one unless you used something like an anamorphic lens, which stretches the pixels when the image distortion is countered in post.